In some instances it is desirable to provide in-service examination of large pressure vessels to verify the integrity of the vessel or to discover any incipient defects so that appropriate repairs can be made before failure occurs.
In some cases, for example, where the vessel is relatively inaccessable or hazardous to humans, the use of conventional inspection techniques and equipment is precluded. A noteable example is a pressure vessel containment for a nuclear reactor. Such a pressure vessel may be in the order of 60 feet in height and 20 feet in diameter with walls of steel 4 to 12 inches in thickness. In use, such a vessel is exposed to radioactive fields. Furthermore, such vessels are ordinarily closely surrounded by heat insulation and a biological shield wall. These and other factors dictate the provision of remotely operable equipment for the in-service inspection of such vessels.
A method for on-site examination of a nuclear reactor pressure vessel has been described by L. B. Gross et al in U.S. Pat. No. 3,715,914 wherein ultrasonic signal transducers for examining the vessel walls are mounted on a carriage which is movably mounted on tracks at the top and bottom of the sides of the vessel. Use of such an arrangement is difficult unless the surface to be examined is relatively free of obstruction, particularly when there is restricted space between the vessel and surrounding structure such as a shield wall. In many cases a number of pipes of various sizes penetrate the vessel walls, such as feed lines, recirculation lines and the like. A tracked arrangement is not well suited for circumventing such obstructions nor for scanning the area above and below such obstructions.
A tank or vessel scaler device which does not require guide tracks is described by T. E. Cowell et al. in U.S. Pat. No. 3,811,320 wherein independently driven cross-members are selectively held on the vessel wall by selectively actuatable electromagnets whereby the device can be moved stepwise across the vessel surface.
Various vehicles which utilize magnetism for adherence of the vehicle to surfaces have been proposed for a variety of purposes and are shown, for example, by J. D. Hilliard in U.S. Pat. No. 1,262,341; by J. W. Allen in U.S. Pat. No. 2,030,840; by Michito Hiraoku in U.S. Pat. No. 3,777,834.
In general, prior devices are found inadequate or limited in their capability for providing the desired degree of accuracy, versatility or ease of operation in the in-service examination of restricted access vessels.
It is an object of the present invention to provide a system for the remotely controlled examination of the walls of a vessel including a vehicle which may be propelled, around obstacles where necessary, to any position on the vessel and wherein the position of the vehicle is determined with a high degree of accuracy.